It is well known that nucleic acids play a key role in cellular functions. Chemical alterations in nucleic acids by UV-, X-, and gamma-ray radiation produce mutagenic and lethal effects. Therefore, in order to understand the action of ultrasound on biological systems, this study is necessary to define ultrasonic toxicity on a molecular level, to set up a safety standard, and to design methods of protection against and repair of these changes if judged harmful. This research project has been following a dual course: 1) the study of the sonochemical changes in nucleic acid components at relatively low intensities of sonication (0.5 - 5 W/cm2, 1 MHz) and 2) the examination of physical changes in T7, phi x, and SV 40 DNAs and of the biological effects on H. Influenzae transforming DNA. By measuring the decrease of their characteristic ultraviolet absorbancies and by analyzing TLC, HPLC, and radioautography of the time-dependent formation of various sonoproducts, dose-response studies of uridine, thymidine, cytidine, adenosine, and guanosine, etc. indicate ultrasound causes profound chemical changes of these nuclueosides in aqueous solutions. On the other hand, our physical and biological studies with isolated DNA indicate that under our experimental condition sonication of DNA produces little physical and biological effects. These results suggest that chemical changes may occur in transforming DNA prior to the loss of its viability.